In a digital demodulation apparatus, there is a limit to the dynamic range of an analog-to-digital (A/D) converter for converting an analog signal to a digital signal.
For this reason, a digital signal converted exceeding the dynamic range of the A/D converter will be output in a form including signal distortion.
Further, in a system where a large number of apparatuses communicate as in a wireless LAN system or other wireless communication system, the reception signal levels will differ largely according to transmission outputs of the individual apparatuses and the distance between apparatuses.
Accordingly, in the wireless communication system, it is necessary to adjust the reception signal level within the dynamic range of the A/D converter. For this reason, a synchronization demodulation apparatus of a wireless communication system mounts an automatic gain control (AGC) circuit as a circuit for adjusting the reception signal level to within the dynamic range of the A/D converter.
Further, in the wireless communication system, a burst signal referred to as a preamble signal is transmitted inserted at the header of the modulated signal.
Further, the AGC circuit mounted in the synchronization demodulation apparatus synchronizes the timing to within the period of this burst signal, while controls the amplification gain based on the received level of the burst signal.
In this way, various circuits have been proposed as the AGC circuit able to be applied to a wireless communication system using so-called burst synchronization.
FIG. 1 is a block diagram of an example of the configuration of a demodulation apparatus mounting a conventional AGC circuit able to be applied to a wireless communication system using burst synchronization.
The apparatus of FIG. 1 is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-205278.
This demodulation apparatus 10, as shown in FIG. 1, is comprised of an automatic gain control amplifier (AGCAMP) 101, A/D converter (ADC) 102, OFDM demodulator (DEMOD) 103, delay unit (DLY) 104, burst detector (BDT) 105, packet detector (PDT) 106, and amplification gain controller (AGCTL) 107.
In a demodulation apparatus 10 having such a configuration, an OFDM reception signal RS received by a not illustrated antenna is input to the automatic gain control amplifier 101.
In the automatic gain control amplifier 101, the reception signal RS is automatically controlled in gain and output as the optimum signal level to the A/D converter 102. Note that, the automatic gain control amplifier 101 controls the case of automatic gain control by a control signal S107 by the amplification gain controller 107 and the case where the control gain is fixed.
The A/D converter 102 converts the input reception signal from an analog signal to digital signal and outputs a digital reception signal S102 to the OFDM demodulator 103, delay unit 104, and burst detector 105.
The OFDM demodulator 103 applies a discrete Fourier transform to the digital reception signal S102 based on the output of the burst detector 105 to demodulate the OFDM signal and outputs this demodulated signal S103 to the packet detector 106 and following processing circuit.
The delay unit 104 delays the digital reception signal S102 by the amount of the burst period and outputs the result as the signal S104 to the burst detector 105.
The burst detector 105 establishes correlation between the digital reception signal S102 and the delay signal S104, detects the burst signal of the period determined by the communication system, and outputs the detected result as the signal S105 to the OFDM demodulator 103 and the amplification gain controller 107.
Further, the packet detector 106 detects a unique word of the packet header from the demodulation signal S103 by the OFDM demodulator 103, detects whether or not the packets were correctly demodulated, detects the end time of the packets, and outputs the detected result as the signal S106 to the amplification gain controller 107.
The amplification gain controller 107 judges whether or not to fix the automatic gain control of the automatic gain control amplifier 101 based on the output signal S105 by the burst detector 105 and the output signal S106 of the packet detector 106 and outputs the judged result as the control signal S107 to the automatic gain control amplifier 101.
In this way, the demodulation apparatus of FIG. 1 fixes or changes the control gain in the automatic gain control amplifier 101 according to whether or not the burst synchronization was established in the burst synchronization system, so it is suitable for a burst synchronization communication system for communication dividing the timing of the burst synchronization and the timing of the data and/or packets.
The OFDM modulation method is a modulation method applies an inverse Fourier transform to 2n primary modulated (QPSK, 16ASAM, etc.) transmitted signal symbols to form 2n number of sub carriers orthogonal to each other on the frequency axis.
The OFDM modulation signal of the OFDM modulation method is a combined signal of a plurality of modulation waves, so the ratio of the peak amplitude to the mean amplitude is large and the fluctuation in amplitude is large.
Accordingly, when applying the conventional automatic gain control circuit explained above to a communication system with abrupt changes in data such as an OFDM reception signal, in order to suppress an occurrence of the circuit distortion due to the pull-in operation of gain following individual data changes, a time constant of the automatic gain control circuit is prolonged.
For this reason, in the conventional automatic gain control circuit, if the period of the burst synchronization is short, the optimum gain can no longer be pulled into.
As a result of this, there is the disadvantage that the optimum gain is gradually pulled into over several packets, error occurs in the header packet, and deterioration of communication efficiency due to resending of data or the like is induced.
Further, in a system wherein a large number of apparatuses communicate such as in a wireless LAN system, the reception signal levels differ largely according to the transmitted outputs of the individual apparatuses and the distance between apparatuses. Therefore, in an automatic gain control circuit having a long time constant, the optimum gain is not pulled into for the individual packets, but the mean gain of the optimum gains of all packets is pulled into and therefore there is the possibility that the system will end up crashing.